The present invention relates to a signal detection system for use in an optical reproducing apparatus in which a light beam emitted from a light source is made incident by means of an objective lens upon a record medium having recorded thereon an information signal along one or more tracks and in which the light beam modulated by the information signal and collected by the objective lens is received by a light detector arranged in a far field of the track of the record medium and comprising four light receiving regions divided about an optical axis of the incident light beam in orthogonal directions, one direction being in parallel with a track direction and the other direction being perpendicular to the track direction.
Such an information reproducing apparatus has been known and has been advantageously applied to an apparatus in which a scanning light spot is projected by an objective lens onto information tracks formed spirally or concentrically in a disc-shaped record medium to read information recorded along the tracks.
In an apparatus for reproducing or picking-up an information signal from the above mentioned record medium, the record medium is usually a digital video disc, a digital audio disc or a digital data disc in which encoded video, audio and/or data signals are recorded as optical information such as optical transmittivity, reflection and phase properties. While such a disc is rotated at a high speed such as thirty revolutions per second, i.e. 1,800 rpm, a laser beam emitted from a laser light source is focussed on the tracks of the disc as a light spot and the optical information is read out by detecting the reflected light beam modulated by the information. One of important properties of such a record medium is a very high density of recorded information and thus a width of the information track is very narrow and a space between successive tracks, i.e. a track pitch is also very narrow. In a typical video disc, a pitch of the tracks amounts only to 1.6 .mu.m. Therefore, the diameter of light spot should be correspondingly small such as 1 to 2 .mu.m. In order to pick-up correctly the recorded information from such tracks having very narrow width and pitch, an error in a distance between the objective lens and the tracks, i.e. a focussing error should be reduced to as little as possible to make a spot diameter as small as possible.
To this end, the apparatus is provided with a focussing servo control system in which an amount and a direction of a de-focussed condition of the objective lens with respect to the disc information surface are detected to produce a focussing error signal and the objective lens is moved in a direction of the optical axis of the objective lens in accordance with the detected focussing error signal.
Furthermore, during reproduction, the light spot should follow the track precisely. For this purpose, the reproducing apparatus is also provided with a tracking servo control system in which an error in a position of the light spot with respect to the track, i.e. a tracking error is detected to produce a tracking error signal and the light spot is moved in a direction perpendicular to the track, i.e. a radial direction of the disc in accordance with the detected tracking error signal.
As described above, in order to compensate the mutual deviation between the beam spot and the information track, both the focussing error signal and the tracking error signal must be detected.
FIGS. 1 to 3 show a known optical reproducing apparatus disclosed in Japanese patent application Laid-open Publication No. 93,223/77. FIG. 1 is a schematic view showing an optical system of the known optical reproducing apparatus. A disc 1 is rotated by a spindle 2 at a speed of, for instance, 1,800 rpm. In the disc 1 there are recorded an information signal along a spiral track or a number of concentric tracks 3. A light beam such as a laser beam emitted from a light source 4 is made incident upon the track 3 of the disc 1 as a small light spot by means of a lens 5, a half mirror 6, a reflection mirror 7 and an objective lens 8. A light flux reflected by the disc 1 is collected by the objective lens 8 and is made incident upon a light detector 10 by means of the reflection mirror 7, the half mirror 6 and a lens 9. The light detector 10 comprises four light receiving regions 11 to 14 divided in orthogonal directions, i.e. a first direction Y parallel to a track direction and a second direction X perpendicular to the track direction as depicted in FIG. 2. The light detector 10 is arranged at a far field of the information track 3. That is to say, the light receiving regions 11 to 14 are arranged sufficiently remote from an image of a pit construction of the information track 3 formed by the objective lens 8, so that they can separately detect diffracted beams of various orders produced by the pit construction.
As illustrated in FIG. 2, output signals S.sub.1 and S.sub.3 from the first and third regions 11 and 13 diagonally aligned are supplied to a first adder 15 to produce a first sum S.sub.5 =S.sub.1 +S.sub.3. Similarly output signals S.sub.2 and S.sub.4 from the diagonally aligned second and fourth regions 12 and 14 are supplied to a second adder 16 to generate a second sum S.sub.6 =S.sub.2 +S.sub.4. Then, a sum signal S.sub.7 =S.sub.5 +S.sub.6 is obtained by a third adder 17 and a difference signal S.sub.8 =S.sub.5 -S.sub.6 is generated by a subtractor 18. The sum signal S.sub.7 is shown in FIG. 3A and constitutes a reproduced information signal which is modulated by the pit construction of the information track 3 and thus has a pit frequency. The difference signal S.sub.8 is shown in FIGS. 3B and 3C and is shifted in phase by .pi./2 with respect to the information signal S.sub.7. The shifting direction is dependent upon a direction of deviation of the light spot from the information track 3. That is to say, in FIG. 3B, the light spot deviates leftward, whereas in FIG. 3C, the light spot deviates rightward. Therefore, by delaying the sum signal S.sub.7 by .pi./2 in a .pi./2 phase shifter 19 to produce a delayed sum signal S'.sub.7 shown in FIG. 3A and then multiplying the delayed sum signal S'.sub.7 with the difference signal S.sub.8 in a multiplier 20, it is possible to generate at an output terminal 21 a tracking error signal having a polarity denoting the direction of the deviation of light spot with respect to the track and an amplitude representing an amount of the deviation. The operational principle of this known method for deriving the tracking error signal has been fully described in the abovementioned Japanese patent application Laid-open Publication No. 93,223/77 and thus, is not explained here more in detail. At any rate, the phase deviation of .+-.(.pi./2) will appear in the difference signal S.sub.8 with respect to the sum signal S.sub.7 in dependence upon the direction of the beam spot with respect to the track. In this known detection system, the phase of the sum signal S.sub.7 is delayed by .pi./2 by means of the phase shifter 19 which may be simply constituted by a low pass filter having a phase delay of .pi./2 at a center frequency of the sum signal S.sub.7, i.e. the pit frequency. However, in such a low pass filter, when the frequency of the sum signal S.sub.7 fluctuates, the amount of delay is changed from .pi./2 and thus, the tracking error signal cannot be derived accurately. Such a drawback could be avoided by providing a phase locked loop for keeping the phase delay to be always .pi./2 for all frequencies of the sum signal S.sub.7. However, such a phase locked loop is very complicated in construction and expensive in cost.